rk structure, and x is from 2 to 3.5 at 50% relative humidity, said mineral being characterized by a d spacing at said humidity within the range which extends from a lower limit of about 10 Angstroms to an upper limit of about 15 Angstroms when A is monovalent and an upper limit of 18 Angstroms when A is divalent or trivalent, with intermediate values possible when A is a mixture of monovalent, di